The exploration, drilling and production of hydrocarbons involves the use of land-based or offshore-located drilling rigs. The drilling rigs consist, for example, of a derrick with a hoisting system for lifting and lowering pipes and other equipment down to, and down into, the well. The hoisting system usually comprises wire or cable, a so-called drill line, a plurality of sheaves, a winch, a deadline anchor, and a storage drum, and is usually connected to sheaves on lifting devices in the derrick.
NO 301384 and NO 303029 describe, for example, the so-called Ram Rig™ concept which comprises a derrick comprising two hydraulic piston-cylinder arrangements for raising and lowering the drill string that is held in the derrick. The cylinders operate between the drill floor and a yoke which travels on guide rails in the derrick itself. A system of this kind makes it possible to position the drill floor at a higher level than the platform floor. The derrick can also be constructed with a significantly lower air resistance, and a higher safety level and a longer lifetime are attained for the most costly components of the derrick. The yoke more specifically comprises at least two sheaves to guide a respective wire, where the sheaves are rotatably attached to a rigid connection between the sheaves, and the rigid connection is rotatably connected to the upper end of each piston-cylinder arrangement. The rigid connection further consists of a beam running between the sheaves and two rotatable arms that extend obliquely upwards from the beam to the top end of each piston-cylinder arrangement.
NO 160387 describes a yoke for use in a derrick where the yokes are, however, operated by gear wheels in engagement with rack rails.
U.S. Pat. No. 4,027,854 describes a more detailed embodiment of a yoke for use in a derrick structure where the yoke is attached to a plurality of piston-cylinder arrangements via which the yoke is raised and lowered. The yoke is suspended in a crosshead beam via a central fastening bolt about which the yoke can pivot to offset the difference in lifting force and stroke length between the piston-cylinder arrangements. The yoke is, however, attached relatively rigidly to the piston rods in the piston-cylinder arrangements so that a misalignment of the yoke when pivoting about the central fastening bolt may result in large shear forces being exerted on the piston rods.
U.S. Pat. No. 4,885,213 describes a system for preventing a misalignment between different components in the system by seeking to achieve a uniform symmetric distribution of the forces on the components. While the objective of U.S. Pat. No. 4,885,213 is to prevent a misalignment, it nowhere describes a solution where the configuration of the device itself provides for a self-alignment.
A disadvantage of the prior art is that if the piston-cylinder arrangements lift the yoke operate unequally (e.g., different stroke of different cylinders), a displacement/uneven distribution of force on the beam (or the different points of attachment on which the forces act) against which the piston cylinders thrust results, and thereby an unequal distribution of force in different parts of the guide arrangement in which the yoke runs. Such a situation can, for example, result in damage to the equipment.